This proposal includes three new projects addressing the mechanical "cochlear amplifiers" of the inner ears of vertebrates. The first project investigates the effects of various noxious agents on the responses to sound of the basilar membrane, the outer hair cells and auditory-nerve fibers at basal sites of the chinchilla cochlea. These agents have been chosen because they target the somatic electromotility of outer hair ceils (the putative amplifiers of mammalian cochleae) in vitro, affect otoacoustic emissions and/or otherwise affect outer hair cell function in-vivo or in-vitro. The second project will extend our recent findings on the development of passive cochlear traveling waves in newborn Mongolian gerbils by studying the maturation of the cochlear amplifier as reflected in the growth of basilar-membrane sensitivity and the strength of compressive nonlinearities. The third project consists of mechanical and electrophysiological recordings in the basilar papillae (analogues of the mammalian cochlea) of avian and reptilian species (pigeon, alligator and turtle), which have basilar membranes but not outer hair cells. The goal is to ascertain whether basilar-membrane vibrations in these species reflect the presence of stereociliar cochlear amplifiers which may play a role equivalent to that of outer hair cell somatic electromotility in mammalian cochleae. The results of the proposed research should be of fundamental value in defining the mechanical bases of auditory transduction in the inner ears of mammals and other vertebrates and may also serve to clarify the pathophysiology of sensorineural hearing loss in humans.